Metal silicides have come into use in the manufacture of advanced FET devices, in order to limit the increase in sheet resistance as processing linewidths decrease. In particular, the silicon in the upper portion of the gate structure of an FET and the area of silicon in the source/drain region adjacent the gate are often converted to a silicide. In a typical silicidation process a metal layer is first deposited over the FET structure, after which the structure is annealed to cause formation of a silicide where the metal is in contact with silicon; unreacted metal is subsequently removed. FIG. 1A illustrates a FET structure 10 formed on a substrate 1 and having a pair of FET gate structures (having gate material 11, 12 over gate dielectric 13, 14) with spacers 15, 16 on the sides of the gate material. A blanket metal layer 17 is deposited on the structure; heat treatment will cause metal in layer 17 to combine with gate material 11, 12 and silicon in the source/drain regions 2 of the substrate. Regions of silicide material 18 are thus formed as shown in FIG. 1B.
Nickel silicide offers a less severe increase in sheet resistance with decreasing linewidth when compared with other metal silicides (e.g. cobalt silicide). For this reason nickel silicide is widely considered to be the silicide material of choice in the manufacture of FET structures where the gate linewidth is 65 nm or below. The temperature in the silicidation heat treatment process is controlled so that the low-sheet-resistance phase NiSi is formed, as opposed to the high-resistance NiSi2.
Unfortunately, the conventional nickel silicide formation process is susceptible to so-called pipe defects—unwanted outward growths of silicide in the substrate, particularly sideways growth under the spacers. Pipes 19 are schematically illustrated in FIG. 1B. The material in the pipes may be either NiSi or NiSi2.
It is known that the density of pipe defects in nickel silicide is influenced by the type of metal deposition process that is used. Deposition processes (and the tools in which they are performed) are generally either directional (collimated) or non-directional (non-collimated), Each deposition process results in a distinct profile of the resulting silicide. Profiles of deposited metal on a pair of gates (after the spacers 15, 16 have undergone a height-reducing etching or “pulldown” process) are illustrated in FIGS. 2A and 2B. A collimated metal deposition process, in which the metal atoms travel in substantially straight paths normal to the substrate surface, causes a buildup of metal 21 on the top surface of the gate structure and substantially uniform metal coverage on the substrate surface 21b (including the surface between the gates); only a thin layer of metal is deposited in region 21a on the side walls of the gate structure (FIG. 2A). In contrast, a non-collimated metal deposition process causes metal 22 to deposit on both the top and sides of the gate structure, so that the metal thickness on the side walls in region 22a is approximately that of the metal on top of the gate (FIG. 2B). In the non-collimated process the gate structure, spacers and deposited metal on top of the gate structure cause shadowing of the substrate close to the spacers, so that metal coverage on the surface (e.g. in region 22b) may be incomplete. This effect is more pronounced as the aspect ratio of the region increases (e.g. as the distance between gate structures decreases).
Profiles of silicides formed as a result of these processes are illustrated in FIGS. 2C and 2D. When a collimated deposition process is used (see FIG. 2C), the nickel silicide 23 has a reduced thickness in region 23a on the sides of the gate structure, and uniform coverage adjacent to the spacers in the source/drain region 23b. In contrast, when a non-collimated deposition process is used (see FIG. 2D), the nickel silicide 24 has greater thickness in region 22a on the side of the gate but poor coverage in region 22b adjacent to the spacers.
Studies have shown that the density of pipe defects is affected by the degree of directionality of the deposition process. For example, deposition of Ni in a non-directional (non-collimated) process may result in a lower nickel silicide pipe defect density than deposition of a similar thickness of nickel in a directional (collimated) process. In addition, numerous workers in the field have noted that the silicide formed after a non-collimated metal deposition has a gate polysilicon sheet resistance about 30% lower compared to silicide formed after a collimated deposition process. This is due to the greater thickness of metal deposited on the side of the gate in the non-collimated process. On the other hand, in the non-collimated metal deposition process the poor coverage of silicide in the source/drain region leads to high contact resistance in that region.
It therefore is highly desirable to combine the best features of the collimated and non-collimated nickel deposition processes. Specifically, it is desirable to have non-directional deposition on top of the gate structure while having directional deposition in the source/drain region. One possible approach would be to perform the nickel deposition in two steps: (1) deposit part of the desired thickness in a process chamber using a conventional, non-collimated process; (2) deposit the remaining thickness in another chamber (for example, in an Advanced Low Pressure Source tool from Applied Materials, Inc.) using a collimated process. The two process chambers would need to be linked so that the substrate is not exposed to air. This solution is expensive, both in terms of equipment and increased substrate handling.
In order to realize the potential for NiSi in FET manufacturing, there is a need for a nickel deposition process and tool which integrate directional and non-directional process steps, so that the resulting silicide has minimal sheet resistance and avoids pipe defects.